Listeria monocytogenes (LM) infection generates a robust innate and adaptive immune response. As a result, several attenuated stains of recombinant LM are being investigated as vaccine vehicles. However, compared to infection with live-LM, immunization with attenuated or killed LM vaccines fails to induce protective cell mediated immunity. Potent activation of the innate immune response is necessary for the robust priming and differentiation of a protective CD8 T cell response. The precise cellular mechanisms that drive a productive vs. a poorly-productive immune response are not well understood. Mounting a protective immune response is dependent on the orchestrated movement of cells within lymphoid organs. This movement is carefully regulated by several factors including host-pathogen interactions and the highly organized lymphoid structure. The cellular mechanisms that control the temporal events that lead to the robust activation of the innate immune system that subsequently induce pathogen-specific cell mediated immunity are poorly understood. In particular, the role of host-pathogen interactions and their geographical localization within a lymphoid organ is poorly defined. Understanding how pathogens interact with immune cells in vivo is important to develop effective and safe microbial based vaccines. Here, we will use distinct immunization models to test the overall hypothesis that changes in the cellular niche of Listeria control the temporal compartmentalization of the bacteria in the spleen. The splenic localization of LM in turn regulates innate immune cell recruitment, local migration and differentiation. The proper progression of these hierarchical events ultimately dictates the outcome of the protective adaptive immunity rendered against the pathogen. In this proposal we will visualize an effective vs. an ineffective anti-microbial immune response with the goal of identifying the pivotal checkpoint(s) of an immune response that lead to protective immunity in vivo. Based on our preliminary data and published studies we propose the following aims: 1) To determine the cellular tropism, localization and antigen presentation after immunization with live-LM or attenuated LM strains. 2) To determine the anatomical mechanisms surrounding ineffectual vaccination. 3) To Determine the cellular mechanisms for the splenic trafficking of live-LM and bacterial antigens in vivo.